Tower congestion 'pushes' calls off the network?

Your iPhone is not the only phone using the network at any given time (to state the obvious), therefore when you initially initiate a call you may display relatively decent signal strength (accurately or not given Apple's recent statement). However, as newer calls are initiated and accepted on to the cell tower, this congestion/bottleneck causes your 'established' call to be virtually pushed off the network to accommodate the newer call. This may also be further exacerbated by the antennae attenuation phenomenon.

It may be also worth considering the type of SIM that is being used too, in that those with a contract SIM may have priority on the network over and above those with a PAYG SIM in this situation.

This is how towers work. There are a number of call slots and a number of handoff slots. Transitioning from tower to tower places you in a handoff slot. These are best illustrated by thinking about driving down a highway. You need to transition to new towers quickly and frequently. Handoffs get priority.

If I congested tower doesn't have enough slots to accommodate a handover, it'll drop a call. This, paired with low signal quality, are the two most common causes of dropped calls.

I'm not exactly sure where you're going with this, as it's a pretty well documented operation. You can read more about it here:

You're close, but no cigar. When you're on the 3G network, the cell tower can tell how far away you are. If you are using the network on the edge of the Cell's influence and someone else starts a call, then the cell will kick off your data connection to prioritise the incoming call.

When you make calls over the 3G network, this will cause you to drop calls in times of congestion. If you are on the EDGE/GPRS network to make phone calls, the EDGE/GPRS cell towers will quite happily pass you between two or three other cells mid call to make sure that traffic is being routed efficiently and avoid congestion. There is only a finite amount of connections each cell can hold so even with this passing about, sometimes they will become congested and calls will drop.

Congestion and poor 3G coverage will cause most of the problems associated with the iPhone 4, especially the problem with 3G data where people have full strength but no data throughput. Not that many people in the UK have reported those problems, so the issue is, in my eyes anyway, down to AT&T

As I have posted on another thread, I find it difficult to understand how so many people can claim 'definitively' that the new handset is unfit for purpose if the FCC approved it as being fit before it came to market? Will they (the FCC) be accused of being complicit in this "conspiracy" next I wonder?

As I believe you confirm here, dropping calls is an inherent trade off between cell tower bandwidth availability and call prioritization, ANY handset, regardless of manufacturer will demonstrate the symptoms that so many are up in arms about now!

I guess some won't be satisfied until the day that mobile handsets can continue to display 100% signal strength, 100% of the time in 100% of locations that they visit

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